Showing papers on "Nuclear quadrupole resonance published in 1996"

System and method for contraband detection using nuclear quadrupole resonance

Abstract: A system for detecting a target substance within a class of explosives and narcotics containing quadrupolar nuclei through the use of nuclear quadrupole resonance (NQR). The system applies an RF signal to a coil (34) to excite the substance under test. If the target material is present, an NQR signal will be picked up by the same coil. That signal is compared with known NQR signals in frequency and amplitude. A signal is displayed in an appropriate way if a threshold value of the NQR signal is equalled or exceeded. The empty coil is statically tuned by means of adjusting the location or capacitance values, or both, of static tuning capacitors (102) in the coil. The coil is tuned after the specimen is inserted into the coil by means of an auto-tune feature (36). Effective RFI shielding (37) is provided to prevent external contaminating signals from being detected by the coil and to prevent RF signals from escaping from the scanner. The invention also includes the method for performing tests with the system.

A different type of oxygen order in RE Ba2Cu3O(6+x) HTc superconductors with different RE ionic radii

Abstract: Using the copper Nuclear Quadrupole Resonance (NQR) spectra of RE Ba 2 Cu 3 O 6+ x (RE123, REY, Nd, La, 0 ≤ x ≤ 1) we have determined the relative intensities of the resonance lines belonging to the Cu(1) sites in the Y123 structure type with two, three and four nearest neighbor oxygens as a function of the oxygen concentration x . The different intensity distribution for different RE shows that, unlike the case in Y123, where the oxygen ions in the basal plane order preferentially into long chain fragments leading to the formation of the ortho-II structure at 0.5 x x = 0.5 and oxygen vacancies above this value tend to be single, forming near x = 0.5, most probably, the “herringbone” structure. Due to different oxygen order, the Nd123 and La123 become superconducting only at high oxygen contents, x ≥ 0.6, whereas at lower x the systems show the antiferromagnetic order in the Cu(2) planes, detected by the zero field nuclear magnetic resonance spectra of the Cu(2) sites and by muon spin rotation experiments.

NMR Studies of the Normal State of High Temperature Superconductors

Abstract: We review NMR and NQR studies in the normal state of high-T c superconducting cuprates. Emphasis is given on the three major contributions brought by these techniques: the presence of antiferromagnetic fluctuations, the possible justification of a single spin fluid model, and the opening of a pseudogap in the spin excitations for underdoped compounds. Some recent developments are addressed and compared to theoretical models. In particular, quantitative analysis of the nuclear relaxation are detailed for 63 Cu, 89 Y and 17 O nuclei, enabling a comparison with inelastic neutron scattering data, and finally pointing out to a serious difficulty with the one-band approach.

Mine detection by nuclear quadrupole resonance

Abstract: Nuclear quadrupole resonance (NQR) methods of mine detection rely on the observation of radiofrequency (RF) signals from the /sup 14/N nuclei present in the explosive The frequencies of these signals lie between 05 and 6 MHz; they are characteristic of a given explosive and provide not only a positive identification but also an estimate of quantity or depth Unlike the better known technique of nuclear magnetic resonance (NMR), no static magnetic field is needed, so that portable probes can be used Furthermore, signals are only seen in solids or solid-like materials, and because of the highly compound-specific nature of NQR frequencies, there is little, if any interference from other nitrogen-containing materials which may be present in the mine casing or the surrounding terrain The paper discusses the principles of NQR and experimental methods, and its application to mine detection

Existence of Line Nodes in the Superconducting Energy Gap of Antiferromagnetic Superconductor URu2Si2. 101Ru NQR Study.

Abstract: We have carried out 101 Ru NQR measurements of URu 2 Si 2 in zero magnetic field in the temperature, T , range from 0.16 K to 26 K. From the NQR spectrum, the electric quadrupole interaction parameters were determined to be ν Q =5.72 MHz and η =0. The nuclear spin-lattice relaxation rate, 1/ T 1 , decreases drastically below 17.5 K. This decrease is associated with antiferromagnetic ordering. In the superconducting state, 1/ T 1 has no coherence peak just below T c =1.2 K, and decreases in proportion to T 3 with no deviation down to 0.2 K, which indicates the existence of line nodes in the superconducting energy gap.

Two-Dimensional Pulsed EPR Spectroscopy of the Copper Protein Azurin

Abstract: Two-dimensional (2D) pulsed EPR spectroscopy was applied to study the copper ligands in azurins from Pseudomonas aeruginosa, Az(pae), and Alcaligenes species NC1IB 11015, Az(asp), in frozen solutions. While a high-resolution three-dimensional crystal structure is available for Az(pae), only a low-resolution structure has been reported for Az(asp). Az(pae) was studied in the pH range 3.9−7.0 and Az(asp) at a pH of 4.8. Measurements were performed at 9 GHz which is usually within the cancellation condition for the remote nitrogen of imidazole ligands. The main technique was the hyperfine sublevel correlation (HYSCORE) technique. At all pH values investigated the 2D HYSCORE spectra of Az(pae) showed correlations between the nuclear frequencies corresponding to the nuclear quadrupole resonance (NQR) frequencies of the remote nitrogens of the imidazole ligands and the double quantum frequency. The spectra showed additional well-resolved cross peaks which indicate correlations between the NQR frequencies of a w...

Method and apparatus for detecting target species having quadrupolar nuclei by stochastic nuclear quadrupole resonance

Abstract: One aspect of the present invention is a method for detecting a class of target species containing quadrupolar nuclei in a specimen by nuclear quadrupole resonance, comprising: (a) generating a random or pseudo-random train of rf pulses; (b) irradiating the speciment with the train of rf pulses; (c) detecting an NQR signal in response to irradiating the specimen; (d) cross-correlating the NQR signal with the random or pseudo-random train of rf pulses, thereby generating a free induction decay signal; and (e) converting the free induction decay signal into a frequency domain signal. Another aspect of the present invention is an apparatus for carrying out the method of the invention.

NMR Study on Spin Gap in Spin Ladder Sr14Cu24O41

Abstract: We have investigated spin dynamics of both the ladder and the chain sites in Sr 14 Cu 24 O 41 by Cu-NMR and Cu-NQR. From the NMR shift, K spin , and nuclear spin-lattice relaxation time, T 1 , the energy gaps in the spin excitation spectrum are confirmed to be Δ=140 K for the chain site and Δ=470 K for the ladder site for Sr 14 Cu 24 O 41 . Doping effects on the spin gaps and electric field gradient at the Cu sites were also investigated by the substituting of Ca, Y and La for Sr. The spin gap of the ladder site decreases with increasing x for Sr 14- x Ca x Cu 24 O 41 and seems to collapse at around x =13 for Ca substitution.

Monoclinic Spin Hamiltonian Analysis of EPR Spectra of Mn2+ in BiVO4 Single Crystals

Abstract: X-band EPR spectra of Mn 2+ in ferroelastic BiVO 4 single crystals at room temperature taken in three orthogonal planes are analyzed using a two-stage least-squares fitting procedure. The absence of site splitting from the spectra shows unequivocally that Mn 2+ lies on a site with two-fold rotation symmetry, whose two fold axis is parallel to the monoclinic crystal axis. The low symmetry effects arising from the monoclinic site symmetry are evidenced by the observed coincidence of the extrema for transitions between different Zeeman levels along the monoclinic axis, and by the 180° rotational symmetry in the monoclinic plane and a small noncoincidence of the turning points in this plane. In the first stage, the spectra are fitted using the monoclinic form of the electronic Zeeman and fine structure terms (of second and fourth degree) appropriate for the crystallographically determined orientation of the monoclinic axis. In the second stage, fitting of the nuclear hyperfine tensor (A) and the nuclear quadrupole tensor (P) is carried out with fixed values of the electronic Zeeman tensor (g) and fine structure terms, i.e. the tensor (D) and the fourth-degree terms, as obtained in the first stage. The principal axes of g, D, A, and P are found to be non-coincident in the monoclinic plane, as would be generally expected for a monoclinic site.

Direct current superconducting quantum interference device spectrometer for pulsed nuclear magnetic resonance and nuclear quadrupole resonance at frequencies up to 5 MHz

Abstract: A spectrometer based on a dc superconducting quantum interference device (SQUID) has been developed for the direct detection of nuclear magnetic resonance (NMR) or nuclear quadrupole resonance (NQR) at frequencies up to 5 MHz. The sample is coupled to the input coil of the niobium‐based SQUID via a nonresonant superconducting circuit. The flux locked loop involves the direct offset integration technique with additional positive feedback in which the output of the SQUID is coupled directly to a low‐noise preamplifier. Precession of the nuclear quadrupole spins is induced by a magnetic field pulse with the feedback circuit disabled; subsequently, flux locked operation is restored and the SQUID amplifies the signal produced by the nuclear free induction signal. The spectrometer has been used to detect 27Al NQR signals in ruby (Al2O3[Cr3+]) at 359 and 714 kHz.

Method of and apparatus for nuclear quadrupole resonance testing a sample, and pulse sequence for exciting nuclear quadrupole resonance

Abstract: A method of nuclear quadrupole resonance testing a sample comprising a first type substance containing quadrupolar nuclei and a second type substance which may give rise to spurious signals which interfere with response signals from the quadrupolar nuclei, comprises applying a pulse sequence to the sample to excite nuclear quadrupole resonance, the pulse sequence comprising at least one pair of pulses; detecting response signals; and comparing, for the or each such pair, the respective response signals following the two member pulses of the pair; the pulse sequence being such that the respective spurious signals following the two member pulses can be at least partially cancelled by the comparison without the corresponding true quadrupole resonance signals being completely cancelled; and for the or each such pair, the two member pulses being of like phase.

Apparatus and method for reducing the recovery period of a probe in pulsed nuclear quadrupole resonance and nuclear magnetic resonance detection systems by varying the impedance of a load to reduce total Q factor

Abstract: A nuclear quadrupole resonance (NQR) or nuclear magnetic resonance (NMR) apparatus which includes a probe and a variable impedance spectrometer. The probe emits a signal towards a specimen which causes the specimen to produce a resonance signal. After a recovery period from emitting the signal, the probe receives the resonance signal produced by the specimen. The variable impedance spectrometer is connected to the probe and has a total Q factor therewith. The spectrometer causes the total Q factor during the recovery period to be lower than the total Q factor before and after the recovery period by varying the impedance of the spectrometer, without changing the electrical configuration of the probe.

Evaluation of Sternheimer Nuclear Quadrupole Shielding Functions and their Application to the Determination of the 57Fem Nuclear Quadrupole Moment from X‐ray‐Determined Charge Densities

Abstract: The Sternheimer function γ(r) describes the shielding/antishielding of the electric field gradient (EFG) at the nuclear position due to polarization induced in the atomic density by the quadrupolar components of the density distribution. The functions for Fe, Fe 2+ and Fe 3+ have been derived by means of Sternheimer's procedure [Sternheimer (1986). Z. Naturforsch. Teil A, 41, 24-36], using a finite-difference method for solving the radial equations for the perturbed wavefunctions and numerical integration for the calculation of γ(r). The shielding factors R, due to the contributions from the electron density of the atom at the nucleus of which the EFG is being considered (the 'central contributions'), are derived from the γ functions. Results are given for near-Hartree-Fock atomic and ionic wavefunctions [Clementi & Roetti (1974). At. Data Nucl. Data Tables, 14. 177-478]. Contributions to the shielding from the core and valence electrons are separated. Since the X-ray multipole formalism describes a flexible valence shell but uses a frozen core, only y∞ core and R core are used in the calculation of Mossbauer splittings from the experimental charge densities. The effect on the shielding of X-ray-determined radial expansion/contraction of the valence shells [Coppens, Guru Row, Leung, Stevens, Becker & Yang (1979). Acta Cryst. A35, 63-72] is evaluated. The combination of spectroscopic nuclear quadrupole splittings and X-ray charge densities on iron pyrite (FeS 2 ), sodium nitroprusside ([Na 2 Fe(NO)(CN) 5 ].2H 2 O) and [Fe(TPP)(pyridyl) 2 ] leads to unweighted and weighted average values for Q( 57 Fe m ) of 0.12(3) and 0.11 (2) x 10 -28 m 2 , respectively, when the core shielding factors are used.

Nuclear quadrupole coupling constants from the Gaussian density-functional method

Abstract: ~Received 29 September 1995! The reliability of density-functional theory in predicting nuclear quadrupole resonance ~NQR! frequencies and nuclear quadrupole coupling constants was tested for a series of straight-chain chloroalkenes ~vinyl chloride, 2-chloropropane, cis- and trans-1-chloropropene, cis-dichloroethylene, and tetrachloroethylene!. The calculations were performed using the linear combination of Gaussian-type orbitals local-spin-density method and the generalized gradient approximation. The calculated NQR frequencies are in good agreement with experimental data. Less satisfactory agreement is found in the prediction of quadrupole coupling constants. The results show that the calculated NQR frequencies and nuclear quadrupole coupling constants can be used to study the electron distribution in molecular systems. @S1050-2947~96!06905-3# PACS number~s!: 31.15.2p I. INTRODUCTION

Apparatus for and method of nuclear quadrupole testing of a sample

Abstract: Apparatus for Nuclear Quadrupole Resonance testing a remote sample containing a given species of quadrupolar nucleus comprises at least one probe (112) having a given maximum cross-sectional dimension; means (102) for applying excitation to the probe or at least one of the probes to excite nuclear quadrupole resonance for a selected range of distance of the sample from the or one such probe, the selected range being at least one tenth of the given maximum dimension of the probe or said one such probe, the apparatus being adapted to produce a non-uniform field over the selected range; and means (106) for detecting the resonance response signal from the sample via the probe or at least one of the probes; the excitation being such as would generate non-zero resonance response signals at all distances within the selected range.

A novel probe design for pulsed nitrogen‐14 nuclear quadrupole resonance spectrometer

Abstract: A fast recovery probe circuit, which involves an active damping unit, is proposed in this paper. Because of its simplicity and high efficiency in converting radio‐frequency power into an oscillating magnetic field, the suggested probe configuration is suitable for operation at low frequency (below 10 MHz), such as 14N nuclear quadrupole resonance where the ringing of a conventional probe usually lasts quite long.

Cu NMR and NQR Study of CuS

Abstract: We report Cu NMR and NQR studies of the cupric sulfide CuS from 4.2 K to room temperature. We have assigned the site for two distinct 63 Cu NMR lines based on an abrupt change of the Cu NMR spectrum at 55 K due to a structural phase transition for both the powder and the oriented CuS. We found that the Cu nuclear spin-lattice relaxation rate 1/ T 1 is magnetic in origin and highly anisotropic, and that the temperature dependence of 1/ T 1 T is similar to that of the uniform magnetic susceptibility χ 0 ( T ). The orbital contribution to the relaxation rate and the nature of the electron spin correlation at the Cu sites are discussed.

Indirect methods for studying nuclear quadrupole interactions in solids

Abstract: Possibilities of indirect detecting of NQR (nuclear quadrupole resonance) signals in solids are considered. Various modifications of the double NQR method are described. Capabilities of various experimental techniques are compared, and characteristic features of NQR on light integer-spin nuclei are considered. A method for taking account of the frequency shifts due to the static magnetic field is described.

NMR and magnetic susceptibility in superconducting and antiferromagnetic Ga-based cuprates Y1−xCaxSr2Cu2GaO7 (0≤x≤0.3)

Abstract: Magnetic susceptibility and NMR/NQR measurements were performed on Y1−xCaxSr2Cu2GaO7 (x=0, 0.1, 0.2, 0.3). The single phase samples annealed at 600°C under oxygen pressure of 30 MPa are superconductors with Tc=35 K for x=0.2 and x=0.3. In spite of the presence of a small Curie-like term, we show that the spin susceptibility in the normal state increases with Ca doping and reaches the value χspin≈0.9 cm3/Cu-mole, which is comparable to other superconducting cuprates. From the observation of Cu zero-field resonance (AFNR) and susceptibility data the parent compound is classified as 2D antiferromagnet (TN=387 K). The transition from antiferromagnetic insulator to superconductor occurs with increasing concentration of carriers, but extends over several tens percent of Ca. The superconductivity is significantly suppressed by increasing disorder within limits of solubility for Ca. The Ga NQR spectra are narrow in both antiferromagnetic and superconducting regimes, but heavily broadened in the intermediate spin-glass-like domain. From x=0 to x=0.3, the 63Cu quadrupole frequency increases from 24 to 28 MHz due to the charge transfer resulting in superconductivity. Other EFG parameters are not markedly changed from those given in YSr2Cu2GaO7 by Pieper [Physica C190(1992)261].

Local microstructure and the cuprate spin gap puzzle

Abstract: Here we propose that the spin gap properties of the underdoped cuprates may be associated with a local inhomogeneous structure in the CuO2 planes. Experimental evidence for such structure is discussed along with suggestions regarding how such local structure might give rise to a spin gap.

Differential scanning calorimetry studies on structural phase transitions in crystals ( n = 1 - 5, X = Br or Cl)

Abstract: The structural phase transitions in (n=1 - 5, X = Br or Cl) were investigated by differential scanning calorimetry (DSC) between about 130 K and the melting points. All of the compounds showed at least one structural phase transition over the temperature range investigated. The results for the Cl compounds agreed well with the reported data obtained by DSC or adiabatic calorimetry measurements. The transition temperatures of the Br compounds are in good agreement with those determined by nuclear quadrupole resonance measurements. The values of the transition entropies obtained suggest that most of the phase transitions in the Br compounds are of the order - disorder type like those in the Cl analogues. The nature of the phase transitions in the Br compounds is discussed in comparison with that of those in the corresponding Cl compounds. The entropy changes due to the melting of orientational and positional orders in the alkylammonium and tetrachlorozincate ions are estimated.

Structural properties of As2Se3 fibers

Abstract: Nuclear quadrupole resonance (NQR) studies of rapidly-drawn fibers of glassy As 2 Se 3 reveal that significant structural changes occur as a function of draw rate. Pulsed 75 As NQR echo measurements have been performed on freshly prepared and aged samples of glassy As 2 Se 3 fibers at three different draw rates (5, 41 and 75 m/min). The 75 As NQR lineshapes exhibit systematic trends in both the linewidths and line asymmetrics. Fibers drawn at the most rapid rate exhibit a second resolved peak in the NQR lineshape that is attributed to the presence of AsAs bonds even though the glass is nominally stoichiometric. Presumably the AsAs ‘wrong bonds’ are compensated by the presence of SeSe bonds. Annealing the fibers for several hours near 200°C produces changes in the NQR lineshapes but does not result in NQR lineshapes that are indicative of the bulk glass. These results are consistent with measurements of structural relaxation as obtained from differential scanning calorimetry in glassy As 2 Se 3 fibers.

Gapless superconductivity in overdoped Hg system: Cu-NQR study

Abstract: The nuclear quadrupole resonance (NQR) investigation of Cu in the overdoped compounds HgBa2CuOy (Hg1201) with y=4. 06 (Tc=92K) and 4.11 (64K) and HgBa2CaCu2Oy (Hg1212) with y=6.13 (Tc=125K) and 6.21 (117K) was carried out in zero field. From the temperature (T) dependence of the nuclear spin-lattice relaxation time, T1, of63Cu, we discuss a cause of the Tc suppression with decreasing the number of CuO2 layers.

Atomic charges in RBa2Cu3O7 superconductor lattices. Electronic structure of the copper atoms

Abstract: The effective atomic charges for all sites of the RBa 2 Cu 3 Q 7 lattices have been found using our Mossbauer data with the 67 Cu( 67 Zn), 133 Ba( 133 Cs), 155 Eu( 155 Gd) isotopes, our calculations of the lattice electric field gradient (EFG) at the lattice sites, as well as the 17 O NMR data available in literature. The charges show holes placed in the chain oxygen sublattice mainly. The Cu 2+ Sternheimer factor has been estimated and the Cu 2+ valence EFG in YBa 2 Cu 3 O 7 has been found. The z-axes of the total, lattice. and valence EFG tensors for Cu(2) coincide with the crystal axis c. For Cu(1) they coincide with the axes b, a, and a, respectively.

Rapid Measurement of Nutation NQR Spectra in Powders Using an RF Pulse Train

Abstract: Abstract The method of two-dimensional nutation NQR spectroscopy, introduced by Harbison in 1989, has been successfully used for determining of the asymmetry parameter of the EFG tensor for spin-3/2 nuclei in both powdered and monocrystal samples in the absence of an external magnetic field. The inconvenience inherent in the method, however, is the long time required, because data acquisition must be repeated for various RF pulse lengths. We discuss a method to reduce the time of the nutation experiment by using a sequence of identical short RF pulses of length r w and distance τ. It is shown that for an NQR frequency ω0 , frequency offset Δω, and pulse parameters satisfying the relation ω0τ + Δωtw = 2πk (k = 1, 2, 3,..., n) a synchronism of pulse action takes place and the nutation interferogram can be measured \"stroboscopically\" between the pulses. The maximum time saving factor that can be obtained as compared to the conventional nutation experiment is of the order of the number of pulses used in a pulse train. The method has been successfully applied for determination of the asymmetry parameter for one of the two 3 5 Cl sites in polycrystalline 2,4,6-trichloro-1,3,5-triazine (cyanuric chloride).

Nuclear magnetic resonance and electron spins: Some history, ancient and in the making

Abstract: Scaling relationships for Nmr spin-lattice relaxation time T 1 and Gaussian transverse relaxation tme T 2G c reviewed and applied to analyse experiments in the normal state of YBa2Cu4O8 Studies of T 2G in the superconducting state are analysed using the theory of Pines and Wrobel to show a preference for d-state orbital pairing over anisotropic s-state Data on 13C Nmr in the normal and superconducting state of the two dimensional organic conductor (ET)2Cu[N(CN)2]Br are presented and shown to have great similarity to that of the cuprates